1. Field of the Invention
The present invention relates to a digital camera which is provided with a solid state imaging device, and more particularly to a digital camera in which an electronic-charge accumulation-time control program for obtaining a proper exposure is installed.
2. Description of the Related Art
Conventionally, there is known a 35 mm film camera (i.e., a Leica-size film camera) in which a shutter speed program is installed in order to obtain a proper exposure. When designing the shutter speed program, it is necessary to set a camera-shake limit shutter speed, i.e., a minimum shutter speed value to counteract the effects of a camera-shake. The camera-shake limit shutter speed is determined as follows.
First, in a 35 mm film camera, a permissible unsharpness, occurring because of being out-of-focus, is the diameter of the minimum circle of confusion (=60 μm) , in accordance with the JIS standard relating to the depth of field. On the other hand, a permissible image blur, occurring because of a camera-shake, is not regulated, and is usually set at the same level as the diameter of the minimum circle of confusion (=60 μm). Based on experience, if the shutter speed (sec) is less than a reciprocal value of the focal length (mm) of the photographing optical system, the image blur is roughly within a permissible range.
In a digital camera, the parameter corresponding to the shutter speed of a 35 mm film camera is the electronic-charge accumulation-time of the solid state imaging device, such as a CCD; and the camera-shake limit shutter speed is defined as a camera-shake limit electronic-charge accumulation-time. Therefore, when designing an electronic-charge accumulation-time control program for a digital camera, setting a camera-shake limit electronic-charge accumulation-time is required. Conventionally, the setting of the camera-shake limit electronic-charge accumulation-time is carried out in accordance with a conventional way in which a camera-shake limit shutter speed to counteract camera-shake in a 35 mm film camera is set.
That is, in the case of a 35 mm film, the size of the exposure area of one frame image is 36 mm×24 mm, and in a CCD of a digital camera, the size of the light-receiving surface is significantly smaller than that of the Leica-size silver halide film. When a predetermined focal length is set for the photographing optical system of the digital camera, the angle of view of the object image formed on the light receiving surface is determined by the focal length. Therefore, for setting the camera-shake limit electronic-charge accumulation-time in accordance with the conventional way for the 35 mm film camera, when the light receiving surface of the CCD is enlarged to 36 mm×24 mm, it is necessary to obtain a focal length for forming an object image with the same angle of view as that for the enlarged surface. Such a focal length is called a film-converted focal length.
Thus, the film-converted focal length is defined as follows.fc=f×(a/b)wherein fc is the film-converted focal length, f is an actual focal length of the photographing optical system of the digital camera, a is a length of a diagonal line of the exposure area (36 mm×24 mm) of one frame of 35 mm film, and b is a length of a diagonal line of the light receiving surface of the CCD.
That is, conventionally, when designing an electronic-charge accumulation-time control program, the camera-shake limit electronic-charge accumulation-time is set to a reciprocal number (1/fc) of the film-converted focal length fc defined as described above.
Solid state imaging devices such as a CCD used for a digital camera have a lot of kinds of pixel pitch. Generally, the smaller the pixel pitch, the higher the resolution of the photographed image, and the greater the pixel pitch, the lower the resolution of the photographed image. The minimum circle of confusion of a silver halide film camera is a parameter determined without regard to the photographing lens or a resolution of the film, and the photographing lens or the resolution of film is higher than the diameter of the minimum circle of confusion by one figure. Therefore, it is unreasonable to apply equally the diameter of the minimum circle of confusion (=60 μm) to digital cameras having CCDs with various kinds of pixel pitch to set a camera-shake limit electronic-charge accumulation-time. Generally, since the resolution of a digital camera is lower than the resolution of film in a silver halide camera and the diameter of the minimum circle of confusion (=60 μm), a camera-shake limit electronic-charge accumulation-time set based on the minimum circle of confusion is shorter than needed. This means that a degree of freedom in design for the electronic-charge accumulation-time of a digital camera is restricted.